The effect of 10 days of total fasting (energy deprivation) on blood polymorphonuclear granulocyte functions, leukocyte numbers, iron and transferrin levels was evaluated in 14 healthy, normal-weight males. Granulocytes from 7 of the subjects were tested in vitro. A statistically significant depression was noted in their bactericidal capacity against Staph. aureus. The 14 subjects showed a marked decrease in the stainable activity of granulocyte alkaline phosphatase and decreases were noted in plasma iron and serum transferrin levels. The iron saturation of serum transferrin was unchanged. Thus, impairment of granulocyte bactericidal functions may occur secondarily to short-term total energy deprivation, in the absence of iron deficiency.